On the Conversion of Wind-charts into Passage-charts. 345 



the important law, that the dispersive power of a given substance 

 is a property depending rather upon its individual nature than 

 on the more or less dense state of aggregation of its molecules, 

 has thus been established. I reserve to myself to pursue this 

 investigation and to extend it to homologous series. 



Finally, as to the formula that has been proposed, I think I 

 have shown that it fulfils its purpose in the simplest and most 

 comprehensive manner. But as long as it does not rest upon any 

 more direct theoretical basis, it may be regarded as a more or 

 less happy expression of the phenomena. 



On the other hand, the unexpected result to which the fore- 

 going calculations have led, namely, that in isotropic substances 

 the constants of refraction and dispersion are independent of 

 physical constitution, but essentially connected with chemical 

 characteristics, cannot any longer be doubted. The only excep- 

 tion to the above rule is presented by water, at least within cer- 

 tain limits, inasmuch as its index of refraction increases below 4° 

 (according to Jamin and others) as the density diminishes. 



I close this communication with a statement of the indices of 

 refraction of air for each one of Fraunhofer's lines : — 



n A = 1*00029286, 

 w B = 1-00029350, 

 n c = 100029383, 

 n D = 1-00029470, 



Bonn, January 15, 1365. 



n E =1-00029584, 

 n F =1-00029685, 

 n G =1-00029873, 

 n H = 1-00030026. 



XLIV. On the Conversion of Wind-charts into Passage-charts. 

 By Francis Galton, F.R.S* 



THE most direct line between two points of the ocean is 

 seldom the quickest route for sailing-vessels. A compro- 

 mise has always to be made between directness of route on the 

 one hand, and the best chance of propitious winds and currents 

 on the other. Hence it is justly argued that an inquiry into 

 the distribution of the winds over all parts of the ocean is of 

 high national importance to a seafaring people like ourselves. 

 A knowledge of the distribution of the winds would clearly 

 enable a calculation to be made which would show the most 

 suitable passage in any given case. 



But as a matter of fact, no calculations have yet been made 

 upon this basis ; much less have charts been contrived to enable 



* Communicated by the Author, having been read at the Meeting of the 

 British Association, in Section A, on August 24, 1866. 



